If Texas Governor George W. Bush's trifecta primary win on Feb. 29 propels him to the Republican Presidential nomination, Senator John McCain's reform candidacy won't be the only casualty. Increasingly, it looks as if the GOP's ruling elite will react to McCain's insurgency, which was fueled by independents and crossover Democrats, by slamming the door on open primaries. There is growing talk of returning to old-style caucuses or closed primaries restricted to card-carrying Republicans.

Big mistake. As of 1999, only 27% of Americans identified themselves as Republicans. Nearly 40% of the electorate considers itself independent or ''other.'' Party loyalty is waning on both sides, and no GOP standard-bearer has captured more than 42% of the popular vote in the last two Presidential elections. Supply-siders and conservative Christians might have a firm hold on Republican Party strings, but they're no good at picking winners--a point McCain drove home in Virginia on Feb. 28, when he assailed the Religious Right for its ''politics of division.'' Whether they support McCain or not, if GOP moderates want to recapture the White House, they would be better off pushing the party to keep the primaries open.

For two decades, Republicans endorsed open primaries as a way to build a Big Tent by wooing indies and conservative Dems to the nominating process. The hope was that the newcomers would stick with the party for the general election and vote the ticket. The strategy worked, helping the GOP win control of state legislatures across the south.

But that approach might meet its end on May 11, when Republican National Committee leaders meet in Indianapolis to consider changes to its primary system. One proposal would take primary decision-making away from the states and return it to national party leadership--a step toward closed primaries. ''I doubt power will be left with the legislatures,'' says RNC Rules Committee Chairman Tom Sansonetti.

PUSH TO PANDER. The party might have a constitutional right to limit participation in its primaries, but it would be political suicide to turn away the swelling ranks of independents and disaffected Dems. In New Hampshire, Michigan, and South Carolina, the electricity generated by a wide-open race produced record turnout. With pols bemoaning sagging voter participation, limiting nominating contests to die-hard partisans will only depress turnout. Worse, it will compel GOP contenders to pander more to the party's extremes. ''We shouldn't be a party that's afraid of independents,'' says Mark Miller, executive director of the moderate Republican Leadership Council.

Republicans already have begun tightening the screws. On Feb. 29 in Virginia, the GOP required voters to pledge in writing that they wouldn't attend Democratic caucuses in April. The loyalty oath was an effort to discourage outsiders from crashing the GOP party. Judging from the outcry, some Virginians didn't think that was a swell idea for a party extolling individual freedom. Elections officials were bombarded with gripes that the pledge was positively un-American.

Democrats are no better than the GOP Old Guard. In 1980, the Demo-cratic National Committee forced states into closed primaries or caucuses and sued those that didn't comply. ''No one's disenfranchised,'' insists DNC Chair Joseph J. Andrew. ''It's allowed our party to become much more unified.''

Exactly, say RNC leaders. But what price loyalty? A 1996 survey by pollster John Zogby found only 27.5% of Republicans and 31% of Dems felt their parties reflected their views. Says Zogby: ''A party cannot exist as a conservative party or a liberal party in the U.S., especially when more and more voters are independent.''

What's the answer? Opening up the political process beats outrages such as the Mar. 7 California primary, in which Dems and indies can vote in the Republican primary but only GOP ballots count. Is that a road map for ensuring political participation in the future? You be the judge.